1. Field of the Invention
This invention relates to a spinal prosthesis for supporting or replacing all or part of one or more vertebral bodies.
2. Description of the Prior Art
The treatment of spinal fractures and tumours often requires the implantation of a prosthesis capable of substituting for the affected vertebra.
In the absence of any commercially available implants, spinal surgeons used bone cement as a crude spacer. PMMA cement has sufficient compressive strength to replace bone for the space of one vertebra. If more than two vertebrae are involved, then the cement must be reinforced with metal rods which function as compression struts.
Although PMMA cement was widely used in this application, it does have disadvantages. More specifically, it is difficult to prepare a cement bridge which is dimensionally accurately enough to restore correct spinal alignment and to install easily. Inter-operatively, soft tissues have a tendency to fold or curl over the edge of the anterior rim and enter the intervertebral space, thus causing the anterior rim to be insufficiently supported. This can lead to displacement of the cement bridge. There is also a high risk of infection with cement struts. The infection risk is increased partly because of the longevity of the surgery, and because of biological reaction with cements which contain antibiotics. Cements have been developed specifically for this type of procedure, to reduce the incidents of infection. The cement in these procedures is mixed to a dense paste and is fitted into the spine. This then cures in vivo. As it cures it generates heat, risking thermal damage to the neural pathways. A 4 to 12xc2x0 C. rise in tissue temperature at the dural sac has been measured in experiments in cadavers. Although this technique is still in use today, it is confined to end stage patients.
Alternative systems have also been proposed and current commercially available instrumentation includes, among other systems, bracing devices and sophisticating jacking devices. These jacking devices all follow the same basic pattern, although to different levels of sophistication. Each uses opposite handed threaded arrangements to adjust the implant height and can be locked off once the correct height and position is achieved. They only differ in the fixing to the vertebrae, and in the degree of anatomic accuracy. These existing systems have been partially successful, but there have been reports of subsidence or anterior translation with attendant problems of hyperkyphosis and pain. The present invention has therefore been developed to address the problems associated with the existing systems of corporectomy based spinal reconstruction.
According to a first aspect of the present invention there is provided a prosthesis comprising a hollow strut terminated by a pair of end caps which are adapted to engage respective vertebrae, each end cap having a flange which is closely received within and supports a respective end of the hollow strut.
To ensure long term stability and resistance to subsidence, the end caps are preferably substantially kidney shaped so that they approximate to the anatomic shape of the hard outer cortex of the vertebral bodies of the vertebrae.
According to a second aspect of the present invention, there is provided a prosthesis comprising a strut having a pair of end caps which are adapted to engage respective vertebrae, each end cap being substantially kidney shaped in a plane substantially perpendicular to the longitudinal axis of the strut.
Preferably the strut is substantially kidney shaped in cross-section, so that the prosthesis as a whole has a posterior concavity which, when implanted, accommodates the spinal cord.
Any spinal prosthesis is subject to compression, torsion and cyclic fatigue loads. By far the largest component of force is a cyclic compressive component which must be transmitted at the interface between the end plate of the adjacent vertebral body and the prosthesis. The anatomy of the vertebral body end plate is well documented and is not discussed in detail here. Put simply, the end plate is approximately kidney shaped and has a soft cancellous centre and a hard outer cortex of bone capable of withstanding compressive loading. Therefore, in a preferred embodiment of the invention, the end caps have a large cutout which corresponds in shape to the outer rim, and allows contact between bone graft packed inside the implant and the vertebral body end plate, or in cases where the device has been packed with cement, to allow for any excess to escape and be removed.
According to a third aspect of the present invention there is provided a prosthesis comprising a strut having a pair of end caps which each have an end wall which is adapted to engage a respective vertebra, the periphery of each end wall extending further from the strut than the remainder of the end wall.
Preferably, the end caps are a press fit into the hollow strut. Preferably, the leading edge of each flange is chamfered, which assists alignment of the end cap with the hollow strut and facilitates assembly.
Preferably, the strut comprises a tube. The tube may be seam welded or extruded.
Preferably, the tube is made of titanium and may be heat treated. Preferably, the tube has a 0.9 mm thick wall.
The strut may be provided with a plurality of through holes. Preferably, the hole pattern is configured to enable trimming to length to suit individual patient need, such as to allow for a degree of lordoeses and kyphoses.
Preferably, in a region away from the periphery, the end wall is provided with a kidney shaped recess.
Preferably, the recess comprises a through hole which provides access to the interior of the strut.
Preferably, each end cap is provided with a shoulder which is adapted to cover the end of the strut and to prevent the end of the strut damaging the surrounding soft tissue.
Preferably, the prosthesis is stabilized by rods and bone screws. For example, 5 mm diameter transvertebral rods and 6.5 mm diameter bone screws may be used. These screws and rods are preferably based on a top loading collet locking system such as the Webb-Morley system. The screws and rods are preferably used anteriorly to bridge the prosthesis and may also be used posteriorly to further strengthen the total assembly.
Preferably a porous titanium plasma coating is applied to the end caps to enhance bone fixation and to resist anterior translation and rotation of the prosthesis by biting into the subcondral bone. When the prosthesis is implanted and under a compression load, there will be intimate contact between the porous coating and the bone. This helps enhance primary stability and encourages osseointegration.
According to a fourth aspect of the present invention there is provided a prosthesis comprising a strut, the ends of the strut being adapted to engage respective vertebrae, the strut comprising a tube.
Preferably each end of the strut is provided with an end cap which is adapted to engage a respective vertebra.
The wall of the tube may be perforated, but otherwise is preferably continuous to enhance its strength. The tube may, for example, be seam welded or extruded.
It is well known that the human spine is curved along its length substantially in a median plane of the body. Consequently, although the spine looks quite straight when viewed from the front or from behind, when viewed from the side there are a series of curvatures alternating in direction. The cervical and lumbar curves are forwarded convexities and are known as lordoses. The thoracic and sacral curves are forward concavities and are known as kyphoses. As a result of this curvature in the spine, in the adult the vertebral bodies are somewhat wedge-shaped to conform to the shape of the curves. The curvature of the spine in the side to side direction of the human body also occurs and is known as scoliosis. Such curvature can be created locally in the spine by a burst fracture resulting from a side impact.
If a vertebral body replacement prosthesis must be inserted in a region of the spine which is curved, or if it is required to treat such curvature by adjusting the shape of the prosthesis, it is necessary to offset the bone engaging surfaces at the ends of the prosthesis so that they are no longer perpendicular to a plane extending parallel to the longitudinal axis of the prosthesis. The offsetting of the bone engaging surfaces of a prosthesis, which comprises a central strut and a pair of end caps, can be achieved by trimming one or both ends of the strut, so that one or both end caps are offset in the respective end(s) of the strut. A disadvantage of this approach is that offset end caps are less securely connected to the strut.
According to a fifth aspect of the present invention there is provided a vertebral body replacement prosthesis comprising a strut terminated at one end by an end cap which is adapted to engage a vertebra, the end cap having an inwardly directed end which is adapted directed towards the strut and an outwardly directed end, which, in use, is directed towards the vertebra, the outwardly directed end defining a vertebra engaging surface which is offset from a plane perpendicular to the longitudinal axis of the strut.
Preferably a further end cap is provided at the other end of the strut. This end cap may also have a vertebra engaging surface which is offset from a plane perpendicular to the longitudinal axis of the strut.
Preferably the or each end cap is substantially wedge-shaped.
Preferably, in use, the vertebra engaging surface of the or each end cap is offset relative to a plane perpendicular to the coronal plane of a patient in which the prosthesis is implanted. In addition, or instead, in use, the vertebra engaging surface of the or each end cap may be offset relative to a plane perpendicular to the median plane of a patient in which the prosthesis is implanted.
Preferably, the respective offsets are between xe2x88x9210xc2x0 to +10xc2x0. Most preferably, the offsets are between xe2x88x925xc2x0 to +5xc2x0.
It will be appreciated that the various features of the invention which are described in relation to a particular aspect of the invention may also be applicable to the other aspects of the invention.